feat: swipe gesture to open/close sidebar on mobile

Swipe right from the left edge to open the sidebar,
swipe left to close it. Standard mobile UX pattern
with edge detection, vertical drift rejection, and
time-based velocity check.

src/App.tsx

@@ -11,6 +11,7 @@ import { ToolCollapseProvider } from './contexts/ToolCollapseContext';
 import { sessionDisplayName } from './lib/sessionName';
 import { X } from 'lucide-react';
 import { useT } from './hooks/useLocale';
+import { useSwipeSidebar } from './hooks/useSwipeSidebar';
 
 const Chat = lazy(() => import('./components/Chat').then(m => ({ default: m.Chat })));
 
@@ -83,6 +84,7 @@ export default function App() {
 
   const [sidebarOpen, setSidebarOpen] = useState(false);
   const [shortcutsOpen, setShortcutsOpen] = useState(false);
+  useSwipeSidebar(sidebarOpen, () => setSidebarOpen(true), () => setSidebarOpen(false));
   const { notify, soundEnabled, toggleSound } = useNotifications();
   const prevMessageCountRef = useRef(messages.length);